1. Field of the Invention
The invention relates to a method of removing n-butyraldehyde from gas streams. It is characterized by washing with dilute aqueous alkali in a mass transfer apparatus.
2. Discussion of the Background
Gas streams containing n-butyraldehyde occur in the production of n-butyraldehyde. When n-butyraldehyde is distilled and condensed, when tanks are filled with n-butyraldehyde, or when n-butyraldehyde or liquids containing it are used for various purposes, outgoing gas will contain n-butyraldehyde.
In order to meet legal and other requirements for protecting workers and the environment, n-butyraldehyde must be thoroughly removed from gas streams, before they are emitted to the environment. In the "Technische Anleitung zur Reinhaltung der Luft" (English translation, "Technical Instructions for Maintaining Air Quality"), n-butyraldehyde is classified in Class 2, and sometimes in Class 1, the latter due to its ready oxidizability to butyric acid. The emission limits for Class 2 substances are 3 kg/hr, and for Class 1 they are 0.1 kg/hr.
According to Ger. AS No. 12 94 950, vapor-phase aldehydes can be separated out from exhaust gases by absorption in high-boiling solvents.
Further, it is known in the art to condense n-butyraldehyde to 2-ethylhexenal by aldol condensation in the gas phase on solid catalysts (Swift, H. E. et al., J. Catalysts, 15, 407-416 (1969)). The reaction in this case is carried out at 250.degree. C. with conversions of scarcely above 50%, and with the catalyst rapidly becoming inactivated.
According to U.S. Pat. No. 2,684,385, an extractive distillation with sodium hydroxide can be carried out for aldol condensation of vapor phase n-butyraldehyde. For this, n-butyraldehyde must be heated to its boiling point of 75.degree. C. Although an extraction without auxiliary agents is theoretically possible, this method is successful only with large amounts of solubilizing agents. For a conversion of 78%, a column with 60 separating stages is required.
According to Polish Patent No. 99,257, vapor phase n-butyraldehyde is passed at 100.degree. C. through a mixture of 6% aqueous NaOH and a solubilizing agent comprised of alcohol-rich hydrocarbons, whereby the n-butyraldehyde is condensed to 2-ethylhexenal. For workup of the reaction products, a mixture of 2-ethylhexenal/water azeotrope and alcohols is distilled, and this must be subsequently separated.
The known methods involve aldol condensation of vapor-phase n-butyraldehyde to form 2-ethylhexenal. They require high temperatures, and the use of aqueous alkalis. They also require additional solubilizing agents. These agents to some extent complicate the isolation of the 2-ethylhexenal as an economic product. No method is known whereby over 90% of the n-butyraldehyde can be removed from gas streams, with the formation of 2-ethylhexenal, in the absence of solubilizing agents.